God called Abraham to become the father of the faithful.
At the Tower of Babel, GOD divided Noah's descendants into nations. GOD placed angels as rulers over each of these nations.
But GOD foreknew that these angels would be tempted by Satan and quickly become corrupt. The great chess match between GOD and Satan was fully in motion. It appeared as though Satan had established a great lead. The first world had been completely destroyed.
Now, the second world had been divided into weaker nations that could no longer efficiently communicate with one another. If Satan could turn these angel overseers to himself, perhaps he could thwart GOD's promise to destroy Satan's seed.
By the time of Terah, Abraham's father, about a thousand years had passed since the flood. Satan's plan seemed solid.
But Satan didn't know that GOD had reserved one nation—a nation that had yet to exist—for Himself to oversee.
Through the lineage of Noah's son Shem, GOD called Abraham (who was called Abram at the time) for a special mission. While still in the Chaldean city of Ur, GOD spoke to Abraham and told Abraham to leave and go to the land GOD would show him. That land was Canaan.
Later, GOD said to Abraham:
“I am your God; be pleasing before Me and blameless. 2 I will establish My covenant between Me and you, and will multiply you exceedingly.”
(Genesis 17:1b-2 OSB)
All of the nations had their gods—the heavenly creatures that ruled over them. To us today, GOD's statement to Abraham about being Abraham's God may seem routine. After all, we know that GOD is all of our God, as He is the Father of us all.
But in reality, this simple statement communicated much more to Abraham—and everyone who lived during the second world. When you realize that there were real heavenly beings (angels) who were the seen by the Gentile nations as "their gods," then you can better appreciate the message GOD was communicating with Abraham.
Effectively, GOD was telling Abraham, "While all these nations have their gods, you are special. For you, alone, I will be your God. And there is no one greater than or equal to me, so you are very special."
GOD chose Abraham for this mission because He knew Abraham would have faith and would complete the mission. Later, Scripture gives us insight into GOD's thinking about Abraham's faithfulness—in multiple places, actually, but here's one:
17 Then the Lord said, “Shall I hide from Abraham, My servant, what I am about to do? 18 But Abraham shall surely become a great and populous nation, and in him all the nations of the earth shall be blessed. 19 For I know he will order his sons and his house after him. They will keep the way of the Lord, to do righteousness and judgment, that the Lord may bring upon Abraham what He has spoken to him.”
(Genesis 18:17-19 OSB)
Here's the thing, though: Abraham's "sons and house" referred, not to Abraham's physical children, for many of them were not righteous or faithful. Instead, it is those who have the faith of Abraham who are referenced here. Those who walk in the example of Abraham are the recipients of the promises.
The apostle Paul wrote concerning this truth:
For not all who are descended from Israel are Israel. 7 Nor because they are his descendants are they all Abraham’s children. On the contrary, “It is through Isaac that your offspring will be reckoned.” 8 In other words, it is not the children by physical descent who are God’s children, but it is the children of the promise who are regarded as Abraham’s offspring.
(Romans 9:6b-8 NIV)
Abraham became the father of the faithful because the faithful are the heirs of GOD's promises to Abraham.